Unless otherwise stated, by “greater unsaturated hydrocarbons” herein we mean the hydrocarbons which contain triple bond and/or two double bonds.
The process for converting hydrocarbons at a high temperature such as steam-cracking or alternatively catalytic cracking, provide unsaturated hydrocarbons such as for example, ethylene, propylene, butadiene, butene; saturated alkanes such as ethane, propane, butane, as well as lighter compounds such as methane, hydrogen and carbon monoxide, and hydrocarbons boiling in the gasoline range. Thus, the gaseous monoolefinic hydrocarbons with two or more carbon atoms, obtained by these processes also contain a considerable amount of hydrocarbons of greater unsaturation degree, i.e. acetylenes and diolefins. In general, the mainly olefin-containing process stream from these processes contains 0.5%-5.3% of acetylenes and diolefins. Acetylenes and diolefins could reduce the activity of the polymerization catalyst and weaken the physical properties of the polymer. Therefore, only after reducing the contents of acetylenes and diolefins below a definite value, can this gaseous monoolefin be used as monomers useful for synthesizing polymers or copolymers.
At present, the economical and simple method commonly adopted in the prior art is converting these greater unsaturated hydrocarbons into the corresponding monoolefins by catalytically selective hydrogenation. The catalytically selective hydrogenation comprises three types: back-end selective hydrogenation, front-end selective hydrogenation and hydrogenation of the cracked gas. The gas from the outlet of the compressor, beside hydrogen, methane, C2 and C3-fractions, also contains C4 fraction (mainly butadiene) and some C5 diolefins. Because of the quick deactivation of the hydrogenation catalyst caused by the polymer formed from diolefin polymerization, and a large portion of the butadiene was lost on the hydrogenation, the process for selectively hydrogenating the cracked gas is scarcely employed industrially.
By “front-end hydrogenation” and “back-end hydrogenation” are meant the location of “acetylenes hydrogenation reactor” relative to “demethanizer”, the hydrogenation reactor located in front of the demethanizer means front hydrogenation, and behind that means back-end hydrogenation.
The removal of acetylenes by back-end hydrogenation is that, the top process stream of deethanizer (methane, hydrogen and carbon monoxide) and the carbon mono- and dioxide-free stream out of the methanation reactor (methane and hydrogen) are added respectively and quantitatively into the top process stream of deethanizer (C2 fraction only) to remove the acetylenes by selective hydrogenation. Because of hydrogenation sensitivity to excursions in concentrations of acetylene and carbon monoxide during the acetylene removal, the selectivity of C2 hydrogenation catalyst must be adjusted by carefully regulating the addition of hydrogen and carbon monoxide. Moreover, because of the purity of the ethylene product being influenced by the impurities (such as carbon monoxide, methane etc.) introduced along with the hydrogen, and fluctuated now and then, a rectifying section or a second demethanizer must be installed at the downstream ethylene column, to separate out the remaining hydrogen and methane.
The front-end hydrogenation process for acetylenes removal has been emerged since the fifties of the twentieth century. In recent years, because of the Palladium catalyst with promoter, which has high ethylene-selectivity, small amount of green oil formed and great space velocity, etc., has been successfully developed, the front hydrogenation process for acetylenes removal has been adopted in more and more ethylene plants. There exist two types of front-end hydrogenation process, i.e. front deethanizing front-end hydrogenation process, and front depropanizing front-end hydrogenation process. The former is that before passing into demethanizer, the acetylene is removed by selective hydrogenation of the top stream of the front deethanizer (methane, hydrogen, carbon monoxide and C2); and the latter is that before passing into demethanizer, the acetylene and partial propyne, propadiene are removed by selective hydrogenation of the top stream of the front depropanizer (methane, hydrogen, carbon monoxide, C2 and C3). The disadvantage of the front-end hydrogenation process is that a large amount of hydrogen in the process stream and the fluctuations in the carbon monoxide content, lead to the acetylenes being easy to leak from the outlet or the abnormal operation of the reactor. These abnormal phenomena were due to the temperature excursions caused by the sensitivity and activity of the fresh catalyst at the initial start up of the ethylene production plant. Moreover, the separation of hydrogen and methane is performed in the demethanizer system where the energy consumption is higher, so the higher the content of the hydrogen passes through the demethanizer, the higher the energy consumes.
A process for hydrogenation of acetylene in the mixed phase front end has been disclosed in CN 1098709A (May 12, 1994) hereby incorporated by reference. A mixed phase hydrogenation reactor is adopted in said patent application. Said reactor is located at the downstream side of the front depropanizer and at the upstream side of the further separation units such as demethanizer and deethanizer. The advantages of said patent application is: as concerns the mixed phase hydrogenation of acetylene, the front depropanizer upstream is able to provide liquid stream into the mixed phase hydrogenation reactor, said liquid stream is used to wash and cool said reactor, and able to reduce the number of the front-end hydrogenation units to fully hydrogenate the acetylenes. It has been found that said hydrogenation units are better able to tolerate excursions in carbon monoxide and acetylene concentrations and the abnormal phenomena of the depropanizer.
The disadvantages of said patent application are:
1. Because of the mixed phase hydrogenation reactor being located at the downstream side of the front depropanizer, the cooled and partially condensed stream rich in C3 and lighter components passing through the mixed phase hydrogenation reactor, said process can only hydrogenate the lower unsaturated hydrocarbons, but not be able to hydrogenate the greater unsaturated hydrocarbons such as butyne, butadiene etc, thus the amount of hydrogen consumed is limited and a large amount of remaining hydrogen passes into the cryogenic section where the energy consumption being higher. 2. In said patent application, because of the stream, before passing through the front depropanizer, being not hydrotreated, the alkynes and diolefins in the stream are easy to form equipment fouling, thus increase the energy consumption. 3. When said patent application being employed, a series of units must be attached to perform respectively the additional treatments of the separated C3 and higher components for acetylenes and diolefins removal, so the equipment cost and energy consumption of the production, taken as a whole, would be increased.
Therefore, there needs a process for hydrogenating the greater unsaturated hydrocarbon in the front end of the process stream of the olefin production plant, without the above-mentioned defects of the prior art.